In wireless personal area networks (WPAN), common topologies are a ‘star’ network operating in infrastructure mode and a ‘cluster’ network operating in ad hoc mode. In a star network, all devices communicate indirectly with each other via a central device called a coordinator. The coordinator receives data from transmitting devices and forwards the data to receiving devices. In a cluster network, the devices communicate directly with each other.
In such networks, the very first device that starts a network is called the PAN coordinator. As the network evolves, other coordinators can join the network. In this case, a joining coordinator is called a ‘child’ coordinator that joins with an already existing ‘parent’ coordinator.
The operation of such networks can be according to the IEEE 802.15.3 and IEEE 802.15.4 standards, see IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements “Part 15.3: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs),” 2003, and IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—“Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs),” 2003.
Because the signals transmitted by all devices share the same frequency channel, it is necessary to enforce a channel access methodology in order to efficiently utilize the network bandwidth. This can be done with a channel access schedule, which determines when and how terminals can access the channel. The access schedule can be broadcast periodically using a beacon, see FIG. 1.
The beacon 110 specifies network parameters, i.e., transmission rates, logical channels, network identifiers, and the channel access schedule. The period between successive beacon signals is called a superframe 100. The beacon is followed by a contention access period (CAP) 120 and a contention free period (CFP) 130. The beacon defines the start of the contention period, the start of the contention free period, and the access schedule for the contention free period. The beacon can also include other parameters as defined by the IEEE standard.
During the contention period, the devices compete with each other to gain access to the physical channel. Typically, a random access method such as Aloha or CSMA is used. After gaining access, devices transmit on the channel strictly according to the access schedule during the contention free period to guarantee interference free packet transmissions.
In a WPAN, there can be multiple coordinators within the same personal operating space (POS), see the IEEE 802.15.4-2003 standard, incorporated herein be reference. For a beacon enabled WPAN, the beacons sent by different coordinators can conflict with each other, directly or indirectly.
It absolutely necessary that the beacons 110 are received correctly for devices to operate in the WPAN.
FIG. 2 shows an example of direct beacon conflict. Here multiple coordinators 201-202 use the same physical channel, i.e., the same radio frequency band, for sending beacons 110 to device 210. The coordinators are within transmission range 203 of each other. If the coordinators concurrently send beacons 110 such that the transmission periods for two beacons overlap 205, then the multiple beacons collide at device 210 because CSMA/CA is not applied to beacon transmissions under the IEEE 802.15.4 standard. The direct conflict is due to the fact that a child coordinator joins the WPAN by associating a parent coordinator already in the WPAN. The direct conflict can also be caused by other conditions in which two coordinators are within each other's transmission range.
FIG. 3 shows an example of indirect interference. Two coordinators 301-302 choose the same physical channel. However, the two coordinators are out of transmission range with each other. However, device 300 is within range of both coordinators. If both coordinators concurrently send beacons 110, the beacons collide with each other at the device 300. This due to the fact that coordinator 301 is part of the WPAN when coordinator 302 joins. Because coordinator 302 is out of range of coordinator 301, coordinator 302 can chose the same channel as coordinator 301. The device 300 must be within the overlap area 310 to encounter indirect beacon conflict. This causes the device 300 to lose synchronization with its parent coordinator 301 and the device becomes an ‘orphan’ as defined by the standard. Even worse, the device cannot re-join with its parent coordinator through ‘orphan scan’ as specified in the IEEE 802.15.4-2003 standard.
As shown in FIG. 4, for a coordinator 400, direct beacon conflict occurs for devices within transmission range of the coordinator in a direct conflict area 401, and indirect beacon conflict occurs outside of range 401 but within double the transmission range in an indirect conflict area 402.
One solution for this interference problem at the network layer is described by Lee et al., in submission IEEE P802-802.15-04-0101-00-0004b, March 2004. That solution has three explicit collision detection requirements before a beacon can be sent.
Another solution uses ad-hoc beacons, see Marsden et al., in IEEE P802.15-15-04-0093-00-004b, March 2004. A centralized control method uses a relay device to connect two piconet controllers (PNCs) to handle beacon collisions. Another solution is described in U.S. patent application Ser. No. 10/434,948 filed on May 8, 2003.